As known, hardening by quenching in press (or press hardening) consists of heating steel blanks at a sufficiently high temperature to obtain an austenitic transformation, and then hot stamping the blanks by keeping them within the press tool so as to obtain quenched microstructures. According to a variant of the method, a cold pre-stamping can be done on the blanks in advance before heating and press hardening. These blanks can be precoated, for example with aluminum or zinc alloy. In this case, during heating in a furnace, the precoating alloys with the steel substrate by diffusion so as to create a compound providing surface protection of the part against decarburizing and formation of scale. This compound is suited for hot forming.
The resulting parts are in particular used as structural elements in automotive vehicles for providing anti-intrusion or energy absorption functions. Thus, the following can be cited as implementation examples: bumper crossbeams, door or center pillar reinforcements or frame rails. Such press hardened parts can also be used for example for fabricating tools or parts for agricultural machines.
Depending on the composition of the steel and the cooling speed obtained in the press, the mechanical strength can reach a higher or lower level. Thus, the publication EP 2,137,327 discloses a steel composition containing: 0.040%<C<0.100%, 0.80%<Mn<2.00%, Si<0.30%, S<0.005%, P<0.030%, 0.010%≦Al≦0.070%, 0.015%<Nb<0.100%, 0.030%≦Ti≦0.080%, N<0.009%, Cu, Ni, Mo<0.100%, Ca<0.006%, with which a tensile mechanical strength Rm of over 500 MPa can be obtained after press hardening.
The publication FR 2,780,984 discloses a greater strength level being obtained: a steel sheet containing 0.15%<C<0.5%, 0.5%<Mn<3%, 0.1%<Si<0.5%, 0.01%<Cr<1%, Ti<0.2%, Al and P<0.1%, S<0.05%, 0.0005%<B<0.08% enables a strength Rm over 1000 even over 1500 MPa to be obtained.
Such strengths are satisfactory for many applications. However, demands for reducing the energy consumption of vehicles drives the search for even lighter weight vehicles through the use of parts whose mechanical strength would be even higher, meaning whose strength Rm would be over 1800 MPa. Since some parts are painted and undergo a paint baking cycle, this value is to be reached with or without thermal treatment by baking.
Now, such a level of strength is generally associated with a completely or very predominantly martensitic microstructure. It is known that this type of microstructure has a lower resistance to delayed cracking: after press hardening, the fabricated parts can in fact be susceptible to cracking or breaking after some time, under the conjunction of three factors:                a mostly martensitic microstructure;        a sufficient quantity of diffusible hydrogen. This can be introduced during furnace heating of the blanks before the step of hot stamping and press hardening: in fact, water vapor present in the furnace may break down and be adsorbed on the surface of the blank.        the presence of a sufficient level of applied or residual stresses.        
In order to resolve the problem of delayed cracking, rigorously controlling the atmosphere of the reheating furnaces and the conditions of cutting blanks was proposed in order to minimize the level of stresses. Performing thermal post treatments on hot stamped parts was also proposed in order to allow hydrogen degassing. These operations are however constraining for the industry which wants a material that enables avoidance of this risk and overcomes these additional constraints and costs.
Depositing specific coatings on the surface of the steel sheet which reduces hydrogen adsorption was also proposed. However, a simpler process is sought which offers equivalent delayed cracking resistance.